For various reasons associated in particular with greater comfort of use (softness, emollience and the like), current cosmetic compositions are usually in the form of an emulsion of the oil-in-water (O/W) type consisting of an aqueous dispersing continuous phase and an oily dispersed discontinuous phase, or of an emulsion of the water-in-oil (W/O) type consisting of an oily dispersing continuous phase and an aqueous dispersed discontinuous phase. O/W emulsions are the ones most sought in the cosmetics field, since they comprise an aqueous phase as external phase, which gives them, when applied to the skin, a fresher, less greasy and lighter feel than W/O emulsions.
Standard O/W emulsions are generally stabilized with amphiphilic molecules of low molar mass (<5000 g/mol), such as emulsifying surfactants of the alkylglycerol or alkylpolyoxyethylene type. However, these surfactants, when they are in a certain amount, have the drawback of inducing a waxy, heavy feel. In addition, it is sought to have available compositions containing the smallest possible amount of surfactants, since they are occasionally poorly tolerated by sensitive skin.
Moreover, it is known practice to replace surfactants with modified carboxyvinyl polymers, i.e. polymers comprising a hydrophobic portion constituted by a fatty chain, such as copolymers of C10-C30-alkyl acrylate and of acrylic or methacrylic acid, for instance the products sold under the name Pemulen by the company Noveon. However, these polymers lead to emulsions comprising large-sized drops (˜10-15 μm), which are difficult to stabilize when it is desired to obtain fluid textures, this resulting in the phenomenon of creaming of the emulsion. These copolymers are moreover pH-sensitive, and must be formulated at a pH above 6 to benefit from their gelling and emulsifying properties.
Moreover, it is known practice from document EP-A-1 069 142 to use amphiphilic polymers derived from 2-acrylamido-2-methylpropanesulfonic acid (AMPS), these polymers possibly being used as thickeners, emulsifiers, dispersants and suspending agents, especially in the cosmetics field. However, the polymers illustrated in the document do not allow the production of O/W emulsions which simultaneously have cosmetic properties that are pleasant for the user and which are very stable and easy to prepare.
Patent application EP-1 302 190 describes makeup-removing emulsions stabilized with an AMPS-based amphiphilic polymer; Example 1 corresponds to an emulsion comprising 1% emulsifying polymer and 20% makeup-removing fatty phase. During application to the skin, the emulsion breaks and leads to a non-uniform deposit corresponding to “quick-break” textures. Although very advantageous for removing makeup, this behaviour is not favourable to the uniform deposition of active agents on the surface of the skin and does not lead to smooth, fondant textures on the surface of the skin.
Document FR-2 843 695 describes O/W emulsions containing an amphiphilic polymer of non-crosslinked AMPS and an oil content of greater than 40% by weight. However such an amount of oily phase leads to a greasy and shiny effect during application to the skin, which the user may find unacceptable.
Moreover, document FR-2 853 527 relates to O/W emulsions containing a non-crosslinked AMPS-based amphiphilic polymer bearing C6-C15 alkyl chains and a lipophilic emulsifier, and describes compositions whose texture usually remains fluid. However, these polymers cannot afford textures of thicker consistency such as creams other than by using a large amount of oily phase or by introducing an additional hydrophilic gelling agent into the aqueous phase. Moreover, in the examples, the amount of polymer is at least 0.8%.
Document FR-2 853 544 describes O/W emulsions containing a non-crosslinked AMPS-based amphiphilic polymer, comprising specific proportions of hydrophobic and hydrophilic portions depending on the carbon number of the alkyl chain. These emulsions are in the form of fluid textures, and they contain a large amount of polymer. Comparative Example 2 of the patent application comprises a high polymer content (1.75%), which, admittedly, leads to a stable emulsion without the addition of surfactants, but its texture is gelled and quivery, is cosmetically unattractive and is difficult to take up by finger.